well, you can either turn up with them both and say that you intend to leave him, but as it's obvious that he wont stay on his own and you already have his sister with you, they will probably suggest you stay. I guess that places in the group are premium and that they cant just add another in, but as you are staying, you are another adult there and therefore fully responsible for your little girl. I guess the key here is whether they feel that she is a disturbance in the session. It may however be a benefit to have a child who has no speech difficulties present to give the kids a peer to copy.
or, you can ask in advance and risk being turned down.